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Some 160 people have been arrested UK-wide during police operations

A government adviser on immigration has questioned the effectiveness of some recent high-profile operations targeting illegal immigrants.

Dr Martin Ruhs told The Observer: "Some policies have an element of a spectacle - you want to send a signal.

"And those policies aren't necessarily the most effective."

The warning comes after the government defended checks on rail travellers, following complaints people were targeted based on ethnicity.

On Saturday, immigration minister Mark Harper said the checks at London Underground stations on people suspected of being in the UK illegally had not been based on racial profiling but on intelligence.

As well as the spot checks at transport hubs, there were raids on workplaces on Thursday. A Home Office spokesman said there were a total of 160 arrests, 21 of which were at London stations.

The other 139 suspected illegal immigrants and rogue employers were arrested at locations across the country including London, Durham, Manchester and Somerset.

Different people will draw the line in different places over what is acceptable in how you treat peopleDr Martin Ruhs, Migration Advisory Committee

Dr Ruhs, an Oxford University economist who sits on the Migration Advisory Committee, warned against using draconian tactics in dealing with the issue.

"In liberal democracies we don't want to do the kinds of things that are commonplace in Singapore or maybe the Middle East.

"You have to draw the line somewhere. Different people will draw the line in different places over what is acceptable in how you treat people.

"Obviously the government has to do something about irregular immigration, but the issue is more complex than many policymakers believe."

Shadow immigration minister Chris Bryant has called for figures on the number and ethnic background of the people who had been arrested to be released.

But Mr Harper said details on the ethnicity of those questioned had not been recorded, with only the details of their nationality, name and date of birth being noted."

The row follows the Home Office's controversial use of vans in London bearing a message encouraging illegal immigrant to "go home".

Deputy prime minister Nick Clegg - who said no Lib Dem minister had known of the plan in advance - said he was "very surprised" to see vans "driving aimlessly around north London" telling illegal immigrants to go home, and that they were not a "very clever way" of tackling the issue.